One important parameter for defining efficiency in data transmission systems is bandwidth, measured as bits transmitted per second. For data storage channels (e.g., magnetic media such as hard disk drives, audiotape, videotape, etc., or optical media such as optical disks or CD-ROMs, etc.), efficiency is measured by data density in terms of number of bits recorded per T, where T is a fixed spatial interval. The higher the density or bandwidth, the more efficient the data storage or communication system.
Current data storage systems write and read data generally at a fixed rate of 1 bit per unit time T to and from the recording medium. The data is recorded as a binary physical feature on the medium (e.g., direction of magnetization for magnetic recording, reflection on/off [or above/below a certain threshold] for optical storage). The unit time T is related to the spatial measure of how close the minimum separation can be for data transitions on the recording medium, which is limited by the physical characteristics of the medium.
FIG. 1 is a diagram showing data recording density concepts. In a magnetic recording medium such as a hard disk drive (not shown), the magnetic moment for a bit of data may be oriented in a one of two opposite directions of magnetization (e.g., in a first direction for data bit 12, in the opposite direction for data bits 14 and 16). The write head generally records data (e.g., bits 12, 14 and 16) at time intervals of one bit per time period T. The read head outputs a waveform 20 corresponding to the magnetically recorded data. Due to physical limitations in the medium (e.g., maximum recording density and/or minimum dimensions in the medium for recording a bit of information, etc.), the medium may be read at maximum rate of one data bit per time period T. The unit time T corresponds to the data transition interval, because data transitions such as 0→1 transition 22 and 1→0 transition 24 must be separated by a length of time of at least T.
The data transition interval T represents a fundamental limit to the rate or speed at which data may be read and/or transmitted from magnetic recording media. In present-day commercial magnetic recording media (e.g., hard disk drives), data transitions cannot occur closer together than the time interval T, due to limits in the magnetics of the media. For example, under certain conditions, if binary data are read and/or transmitted from magnetic recording media at rates faster than 1/T (i.e., transitions are closer than T units of time apart), higher nonlinear distortion and partial erasure may result, leading to the disappearance of one or more data transitions which may be too close to a previous or subsequent transition.
Data storage systems (hard disk drives, audiotape, videotape, etc., or optical media such as optical disks or CD-ROMs, etc.) generally involve a write head and a read head, respectively acting as a transmitter and receiver for the data. There is a relative movement between the write head or read head and the recording media during write/read operations. These operations are usually achieved by fixing the head position, and moving (e.g., spinning) the medium. For a given velocity ν between the head and the medium, the minimum time interval T between data transitions is determined by minimum temporal transition constant Tt for the medium:T=Tt/ν  (1)
Therefore, for a given velocity ν between the head and the medium, maximizing the recording density is equivalent to maximizing the data write/read rate (in units of bits/second). For conventional hard disk drives, the only apparent way to increase the physical density on the storage medium is to decrease T. Demands for increasing the recording density of magnetic recording media continue nonetheless, and the need for reading magnetically recorded information at rates faster than the current fundamental limit(s) of magnetic recording media has been long felt.